Recently, with the proliferation of wireless communications, radio waves of various wireless systems are present in close frequency bands. Generally, in order to receive a desired signal with a predetermined quality, it is preferable that there is only the desired signal and no interference signals. In other words, when a receiving device receives an interference signal together with the desired signal, the reception precision of the desired signal in the receiving device is deteriorated, and it becomes difficult to correctly reproduce transmitted data.
In regard to such problems, the reception precision of the desired signal can be enhanced by performing an interference-avoidance operation of detecting a frequency band in which an interference signal is present, and shifting the frequency band of the desired signal so as to arrange it in a frequency band that is different from that of the interference signal. However, in this case, since the total frequency bandwidth occupied by the desired signal and the interference signal must at least be greater than or equal to the sum of the frequency bandwidths of the respective signals, this method is not preferable from the point of view of the frequency utilization efficiency.
On the other hand, a technique is proposed in which a multicarrier transmission scheme is introduced that applies a forward error correction code to signals that are adjacent in a frequency domain, overlapping of spectra of transmission signals in the frequency domain is allowed, and then each transmitting device transmits a signal, thereby reducing the total frequency bandwidth occupied by a plurality of systems and increasing the frequency utilization efficiency. In this technique, the receiving device detects the frequency position and band in which an interference signal is present based on any means such as a pilot signal or a null signal interval, performs filtering or equivalent weighting signal processing to suppress multicarrier signals and interference signals in the detected frequency position of the interference signal, demodulates respective subcarriers of a multicarrier signal after suppression by the filtering has been performed, and subjects the demodulated signal to error correction. Thus, it becomes possible to adequately achieve an error correction effect using subcarriers at frequency positions unaffected by the interference signal, restore data of subcarriers affected by the interference signal, and receive the data correctly. Hereinafter, this type of transfer technique will be termed superposed multicarrier transmission.
In this way, in the field of recent wireless communications, there is a demand for shared-frequency wireless communications. FIG. 36 is a conceptual diagram showing the entirety of two wireless local area network (LAN) systems with different frequency channels, as an example of a combination of wireless communication systems sharing a frequency band.
In the figure, the wireless communication systems are provided with wireless LAN base stations 2a and 2b, and a receiving device 1a. The wireless LAN base station 2a performs communications using a frequency band of CH1 having a center frequency fa. On the other hand, the wireless LAN base station 2b performs communications using a frequency band of CH5 having a center frequency fb (where fa<fb).
In this case, the receiving device 1a is arranged at a position at which radio signals of both the wireless LAN base station 2a and the wireless LAN base station 2b arrive, and receives a signal in which two radio signals including a radio signal having the center frequency fa and a radio signal having the center frequency fb partially interfere with each other.
In this way, when the receiving device 1a communicates with the wireless LAN base station 2a serving as its communications target, it is essential for the receiving device 1a to accurately receives a desired signal even in shared-frequency wireless communication in which the transmission frequency band of the desired signal having the center frequency fa partially overlap the transmission frequency band of an interference signal having the center frequency fb from the wireless LAN base station 2b. 
It is noted that as another example of sharing a frequency band, there is a case in which frequencies are shared between systems with different communication schemes, such as combinations of a wireless LAN system, Bluetooth (registered trademark), and WiMAX (registered trademark).
On the other hand, Non-Patent Document 1 gives a detailed description of a mechanism of a turbo code, which is one type of error correction code for accurately receiving a desired signal, and reception performance when the turbo code is combined with orthogonal frequency-division multiplexing (OFDM), which is a multicarrier signal.
Additionally, two-way communication systems sometimes employ a retransmission technique as a compensatory technique when transmitted data has not been properly received. For example, in an automatic repeat request (ARQ) error control scheme, error detection processing is performed at a receiving device, and, when no error is detected, an acknowledgment (ACK) is transmitted to a transmitting device; when an error is detected, a negative acknowledgment (NACK) is transmitted to the transmitting device. The transmitting device, which receives a NACK, retransmits the data corresponding to the NACK until an ACK is received.